1. Field of the Invention
The present invention relates to the generation of an Intravascular Ultrasound (IVUS) image from a mechanically rotating intravascular transducer and, more particularly, to measuring non-uniformity in the angular velocity of a rotating ultrasonic transducer.
2. Discussion of Related Art
Ultrasonic imaging is widely used in medicine. In particular, it can be used for making images from inside body cavities such as the vascular system, and thus aiding in the diagnosis of disease. A probe containing an ultrasonic transducer is inserted into the body area to be imaged. The transducer transmits an acoustic pulse into the body tissues, and detects the reflections of the pulse at tissue boundaries due to differences in acoustic impedance, as well as the back scattered sound from acoustically heterogeneous tissue. The differing times taken for the transducer to receive the reflected or back scattered ultrasound pulses correspond to differing distances of the tissues from the transducer. By stepping or sweeping the transducer through a set of selected angles, a two-dimensional ultrasound image corresponding to a map of the acoustic impedance boundaries or back scattering coefficients may be obtained. From this image, the condition of the body tissues can be determined. For example, the method of Intravascular Ultrasound (IVUS) sequentially transmits ultrasound pulses in equally spaced angular increments around all or part of a circle to obtain cross-sectional images of coronary arteries, thereby demonstrating areas of atherosclerotic plaque, calcification, etc.
Generally, there are two types of ultrasonic probes for IVUS imaging. The first type employs a synthetic aperture technique. For example, U.S. Pat. No. 4,917,097 (Proudian et al.) and U.S. Pat. No. 5,186,177 (O'Donnell et al.) teach how an ultrasonic pulse may be transmitted in a particular direction from a transducer using the method of synthetic aperture. Generally, this involves the sequential excitation of selected elements in an array of transducer elements.
The second type of IVUS probe scans tissue, for example, the tissue of the coronary artery, by the mechanical rotation of a mechanism that emits ultrasonic pulses and detects portions of the emitted pulses that are reflected from the tissue. The mechanically rotated type of probes include a few subclasses. In a first subclass, either a distal (remote from the operator) transducer or a distal mirror is rotated by an extended drive shaft driven by a proximal motor (e.g., U.S. Pat. No. 4,794,931 (Yock) and U.S. Pat. No. 5,000,185 (Yock)).
In a second subclass, the rotation is confined to the distal end, where either a miniature motor (e.g., U.S. Pat. No. 5,240,003 (Lancee et al.) and U.S. Pat. No. 5,176,141 (Bom et al.)) or a fluid driven turbine (e.g., U.S. Pat. No. 5,271,402 (Yeung et al.)) is used to rotate the transducer or mirror.
In a third subclass, a stationary proximal transducer is acoustically coupled to a rotating acoustic waveguide that guides the sound to and from the distal end (e.g., U.S. Pat. No. 5,284,148 (Dias et al.)).
In a fourth subclass, a turbine is rotated by an acoustic signal generated outside the vessel to direct another ultrasonic signal in a rotating fashion (e.g., U.S. Pat. No. 5,509,418 (Lum et al.)).
In a final subclass, an external driving member rotates a tube to rotate a reflecting element at the distal end of the tube to reflect ultrasound (e.g., U.S. Pat. No. 5,507,294 (Lum et al.)).
Presently, probes that direct ultrasonic pulses by mechanical rotation are more widely used than probes that electronically aim the pulses. The mechanical approach can be implemented using a single transducer, while the electronic approach requires an entire array of transducers to be contained in the distal end, which may be difficult to pass into the blood vessel of interest.
One concern in using an IVUS probe with mechanical rotation, however, is that the angular velocity of the rotating structure, e.g., the transducer, that directs the ultrasonic pulses may be non-uniform throughout the structures 360.degree. range of rotation. If the rotating structure changes its angular velocity during each period of rotation (as is usually the case), then adjacent ultrasound pulses will be transmitted at non-uniform angular separations as the structure rotates. Such a system therefore will image tissues at uneven spatial intervals and the image produced by it will appear distorted.
One cause of this non-uniform angular velocity (in a catheter using a drive shaft) is mechanical friction between the spinning drive shaft and the surrounding stationary sheath as they bend through the tortuous path of the blood vessel. Although the proximal (near the operator) end of the drive shaft may be rotating at the desired angular velocity, any bending of the catheter along its length will cause the angular velocity of the distal end of the drive shaft to vary from the desired velocity as the distal end of the drive shaft moves to different positions within its 360-degree range of rotation.
One system that is capable of measuring and correcting the non-uniform angular velocity of IVUS transducers is described in U.S. Pat. No. 5,699,806 (Webb et al.), which is assigned to the assignee of the present application, and which is incorporated herein by reference.
U.S. Pat. No. 5,485,845 (Verdonk et al.) describes a technique for detecting nonuniformity in the angular velocity of an IVUS transducer by using an array of beacons positioned on a sheath in which the transducer is disposed. This method, however, requires special catheters that have delicate structural features added to them to make them function properly.
What is needed, therefore, is an improved technique for measuring the angular velocity of a rotating transducer.